The invention relates to a process for the continuous mass polymerization of olefinically unsaturated monomers.
A process of this kind is particularly useful to produce polymers which are used as a solid, as for instance, as a molding material or as a powdered coating agent, or which are meant to be applied in the form of a solution or dispersion in a solvent which is not suitable as a medium for carrying out a solution polymerization or emulsion polymerization. Instead, these polymers are usually prepared by polymerization in an inert solvent which is subsequently evaporated.
Preparation by mass polymerization would be preferable, but batchwise mass polymerization in a stirred reactor is difficult to accomplish because of problems associated with the removal of the developed heat of polymerization and with the required stirring.
One proposed method to carry out a continuous mass polymerization is by passing the monomers and the required initiator, chain length regulator, and other additives through a long heated tube or by supplying them to an extruder after having been pre-polymerized to a syrup. It is difficult, however, to make a product of a constant quality and homogeneous composition using this method especially if a low monomer content in the product is desired.
In accordance with the present invention a polymer is prepared by heating, under polymerization conditions, a liquid mixture of one or more olefinically unsaturated, polymerizable compounds, at least one polymerization initiator and, if desired, other usual additives. The continuous mass polymerization is accomplished by continuously supplying to a tubular reactor one or more olefinically unsaturated monomers along with a polymerization initiator and, if desired, other commonly used additives. The tubular reactor is provided with internal static mixing elements. The reactor temperature, should range between the melting point of the polymer to be formed and about 300.degree. C., with a pressure ranging between about 2 ats abs. and about 100 ats abs. Between about 50% and about 95% of the reaction mass leaving the tubular reactor is continuously returned as feed to said reactor while the remaining reaction mass is discharged as product. The reaction conditions are predetermined so that the discharged reaction mass has a conversion degree of at least about 75%.
The degree of conversion is understood to mean the percentage of monomer polymerized. If apart from the monomer only a small amount of other compounds is supplied to the reactor, the degree of conversion is almost equal to the weight percentage of polymer in the reaction mass.
Together with the elements required for recirculation of part of the reaction mass, the tubular reactor forms a circulating reactor.
A polymer prepared according to the present invention will contain about 25% or less, preferably less than about 15% of monomer, and is suitable for further processing. This further processing may be conducted after removal of the monomer by, for example, a treatment at elevated temperature and/or decreased pressure. Preferably, however, polymers having a low monomer content of at most about 5% of monomer and more particularly less than about 2% of monomer, are prepared by the process of the present invention. It is possible to prepare such a polymer in one circulating reactor by the process of the present invention. However, the degree of conversion in a first circulating reactor may be limited, for example, to between about 80% and about 90% with this product then fed continuously to a second circulating reactor which is designed and operated analogously to the first circulating reactor. The process with two circulating reactors is preferred since there are more possibilities in that case for variation of the process conditions.
The second circulating reactor is quite similar to the first and is comprised of a tube with internal static mixing elements. The reactor temperature should range between the melting point of the polymer to be formed and about 300.degree. C. with a pressure ranging between about 2 and about 100 ats abs. Means are provided to continuously return about 50 to about 95% of the reaction mass leaving the tube again to the circulating system and to discharge the remaining portion. If desired, additional circulating reactors may be applied in series. The degree of conversion in the second or last reactor in the series is preferably at least about 95% and more particularly at least about 98%.